1. Field of the Invention
The present invention relates generally to improvements in hydraulic pressure release bypass valves, and more particularly, but not by way of limitation, to improvements in bypass valves for constant displacement pumps.
2. Brief Description of the Prior Art
In many applications of hydraulic pumps, it is desired that a pump deliver pressurized hydraulic fluid at a substantially constant rate by volume while the pressure at which the fluid is delivered is permitted to vary. For example, the pump might be used to drive a hydraulic motor at constant speed against a load that varies with time. A constant displacement pump is ideal for such applications. Pumps of this type are constructed to draw in a fixed amount of fluid for each operating cycle of the pump and discharge the same amount of fluid so that the delivery rate remains constant. The pressure at the pump outlet is then determined by the load.
The constant volume at varying pressure characteristics of such pumps can result in a problem should the hydraulic fluid system receiving the hydraulic fluid from the pump become blocked. For example, should the load on a hydraulic motor become large enough to stop the motor, the pump will be operating on a trapped volume of substantially incompressible hydraulic fluid with the effect that the operation of the pump can be violently halted. This halt can damage the pump and, if the pump is driven by an electric motor, burn out the motor. These results are commonly avoided by providing the pump with a bypass valve that diverts fluid being delivered from the pump outlet to the pump inlet should the pressure at the outlet rise to a preselected value.
Prior art bypass valves have, in the past, given rise to a second problem. In many circumstances, the nature of the load on the pump will give rise to pressure surges in the pump outlet that, if unrelieved, will suffice to operate the bypass valve even though no blockage of flow of hydraulic fluid from the pump has occurred. In extreme cases, the resulting bypassing of the system downstream of the pump can result in a cessation of operation of such system with the result that continued operation of the pump merely cycles hydraulic fluid between the inlet and outlet thereof. In less extreme cases, the diversion of fluid from the downstream system to the pump inlet results in a loss of operating efficiency of the pump.